Molecular sieves are ordered, porous crystalline materials having a definite three-dimensional crystal structure, within which there are a large number of small cavities which are interconnected by a number of still smaller channels or pores. These cavities and pores in any specific molecular sieve material are of precisely uniform size. Since the pores are of such size as to accept for adsorption molecules which are small enough to pass through the pores, while rejecting molecules of larger size, the materials have come to be known as "molecular sieves" and are utilized in various ways which take advantage of this property. Molecular sieves may be used, for example, as catalysts, selective adsorbents, drying agents, ion exchange materials, and for other purposes. Aluminosilicate molecular sieves are frequently referred to as zeolites.
The synthetic crystalline aluminosilicate zeolites are the best known molecular sieves. These materials are characterized by a rigid three-dimensional network of SiO.sub.4.sup.- and AlO.sub.4.sup.- tetrahedra, which are cross-linked through shared oxygen atoms. The electronegativity of the aluminum-containing tetrahedra is balanced by the inclusion in the crystal of a cation, typically monovalent or divalent, such as an alkali metal (e.g. sodium) or an alkaline earth metal (e.g. calcium). The monovalent or divalent ion is typically at least partially exchangeable by conventional ion exchange techniques. The aluminum and silicon are not exchangeable. Various aluminosilicate molecular sieves are known. One of these is ZSM-5, which is described, for example, in U.S. Pat. No. 3,702,886 to Argauer et al.
Less well known are the ferrisilicate molecular sieves. One of these, ZSM-12, is described in published European Patent Application (EPA) No. 0013630. Another is the crystalline silicate described in U.S. Pat. No. 4,208,305 to Kouwenhoven et al. This latter material is of the ZSM-5 type and, according to the patent, consists structurally of a three-dimensional network of SiO.sub.4, FeO.sub.4, and optionally AlO.sub.4, GaO.sub.4 and GeO.sub.4 tetrahedra which are interlinked by oxygen atoms. The patent discloses a number of catalytic processes in which the molecular sieves may be used. However, direct conversion of a carbon monoxide-hydrogen mixture to a hydrocarbon mixture (the Fischer-Tropsch synthesis) is not among these reactions.
Iron-containing zeolites are also known. These may be prepared by (a) physical admixture of a zeolite and an iron component, (b) ion exchange of Fe (III) into a zeolite, (c) adsorption of a volatile metal compound in the zeolite cavities followed by thermal decomposition, and (d) impregnation of a zeolite with a solution of a ferric compound followed by thermal decomposition.
In catalysts where the iron component is physically mixed with the zeolite ZSM-5, an intimate mixture between the two components is very difficult to obtain. Thus, all of the iron component is likely to be on the outside of the pores of the molecular sieve making these catalysts least selective for the Fischer-Tropsch reaction. Further, the formation of large metal oxide particles decreases the amount of surface available for reactions to take place.
Loss of crystallinity and thermal stability is reported for synthetic zeolites which are ion exchanged with ferric ions. The ion exchange of Fe (III) cations into the zeolite can give rise to a high dispersion of the iron component. However, ion exchanging of Fe (III) ions into the zeolite ZSM-5 has not been completely successful, due to the size of the hydrated iron complex and the high dispersion of monovalent exchange sites within the zeolites.
Iron (O) (i.e., metallic iron) species can be introduced into the pores of a zeolite by adsorption and subsequent decomposition of the iron complexes. The most common volatile metal compound that is used to prepare iron containing zeolites is iron pentacarbonyl, Fe(CO).sub.5. The size of the iron pentacarbonyl is just about ideal to be adsorbed by zeolite Y. The iron pentacarbonyl is first adsorbed by the zeolite and then the carbon monoxide is driven off from the iron pentacarbonyl by thermolysis. This process of making iron containing zeolites has the disadvantage that during the process of thermolysis, the adsorbed metal compound tends to come out of the pores of the zeolite. Moreover, the iron pentacarbonyl is too large in size to enter the zeolite ZSM-5 and hence when used over zeolite ZSM-5 will have all of the iron present outside the pores of the zeolite ZSM-5.